Solid-state NMR Studies Of Ion-doped Perovskite Materials

Organolead halide perovskites have excellent optoelectronic properties and are one of the key materials used in the new generation of photovoltaic solar cells.At present,the poor structural stability is an important problem that hinders the application of organic-lead-halide perovskite-based solar cells.In this paper,two kinds of doped organolead halide perovskite materials were prepared by doping the different lattice sites of organolead halide perovskite materials.By using solid-state nuclear magnetic resonance(NMR),combined with differential scanning calorimetry,fluorescence spectroscopy and other analytical methods,the prepared doped organolead halide perovskite materials were studied in great detail.The work in this thesis presents a detailed investigation on the structural changes induced by the doping.The correlation between structural transitions and macroscopic properties of the materials was discussed afterwards.The main contents of this thesis are as follows:(1)A detailed ~2H NMR study was performed on the molecular motions of the embedded cations in the cation-doped lead bromide perovskite.By using the selective deuteration of DMA and MA cations,we achieved selective NMR observations of different embedded cations in the materials.The results show that at low temperature,both the DMA and MA cations in the material have the molecular motions close to the double rotation model.With the increase of temperature,the motional freedom of the DMA and MA cations increase,and their motions gradually transform into the fast isotropic motions.It was found that the DMA cations move faster than MA cations at the same temperature,indicating that there is a significant inhomogeneity in the states of the cations in this material.Based on the study of the cation motions,the molecular mechanism of the phase transition of the material was discussed afterwards.(2)A series of I/Br-doped two-dimensional perovskite materials,(PEA)₂Pb Br₄,were prepared.The luminescence of the doped(PEA)₂Pb Br₄ materials was successfully adjusted by the doping.The results show that iodide ion doping can make the luminescence property with obvious red shift,while the chloride ion doping can make the luminescence property with obvious blue shift.At the same time,the chloride ion doping can also make the material have high quantum yield and long fluorescence lifetime.In-depth research on the above materials found that iodide ions and chloride ions have different doping mechanisms for the two-dimensional perovskite(PEA)₂Pb Br₄:The iodide ion doping causes the lattice expansion,whereas the chloride ion doping causes the lattice shrinkage.The work in this part provides a molecular-level explanation for the modulation of the optical properties of materials by halide ion doping.